1. Field of the Invention
The invention relates to ablative insulation, particularly insulation to protect the interior of a rocket motor from the combustion products of burning propellant. More particularly, the present invention relates to low density thermoplastic elastomeric ablative insulation.
2. Technology Review
The combustion of a propellant in a rocket motor creates a hostile environment characterized by extremely high temperature, pressure, and turbulence. The combustion temperature within the motor often exceeds 6,000.degree. F., and the pressure within the motor frequently exceeds 1,000 psi. Gas velocities typically range from Mach 0.2 in the inlet region to Mach 10+ at the aft end of the rocket motor nozzle. This environment is particularly hostile in a solid rocket motor because its combustion gas contains chemical species and particulates which tend to physically and chemically erode exposed rocket motor nozzle components. While the combustion of a rocket propellant is usually brief, the conditions described above can destroy insufficiently protected or inferior rocket motor parts prematurely and jeopardize the mission of the motor.
Parts of a rocket which are exposed to the high temperatures, pressures, and erosive flow conditions generated by the burning propellant must be protected by a layer of insulation. Various materials have been tried as insulation, such as silica dioxide, glass, or carbon fiber reinforced silicone and/or polyisoprene elastomers, but reinforced resin composite materials are most commonly used. These include phenolic resins, epoxy resins, high temperature melamine-formaldehyde coatings, ceramics, polyester resins and the like. These materials, when cured, usually become rigid structures which crack or blister when exposed to the rapid temperature and pressure changes occurring when the propellant is burned.
The best rocket insulation materials previously known to the art are elastomeric polymers reinforced with asbestos, polybenzimidazole fiber, or polyaramid fiber. These compositions are ablative insulation because they are partially consumed during combustion, but nevertheless they provide protection for the rocket motor. Such materials are capable of enduring in a rocket motor long enough to allow complete combustion of the propellant. Asbestos-reinforced elastomeric insulation is the subject of U.S. Pat. No. 3,421,970, to Daley et al., issued Jan. 14, 1969, and U.S. Pat. No. 3,347,047, to Hartz et al., issued October 17, 1967.
Environmental and health concerns have led manufacturers to seek an acceptable replacement for the asbestos in rocket motor case insulation. One alternative elastomeric insulation contains aramid polymer fibers in combination with a powder filler. That insulation is disclosed in U.S. Pat. No. 4,492,779, assigned to Morton Thiokol, Inc., now known as Thiokol Corporation. A third alternative is elastomeric insulation which contains polybenzimidazole polymer fibers in combination with a powder filler. That insulation is disclosed in U.S. Pat. No. 4,600,372, also assigned to Morton Thiokol, Inc. (See also U.S. Pat. No. 4,507,165.)
Another problem with existing rocket motor insulation is the expense and difficulty of fabricating an insulator and installing it, either as one piece or in sections, within a rocket motor casing. The problems of fabricating thermosetting resinous insulation which is not capable of being cast are described in U.S. Pat. No. 3,177,175, issued to Barry, Jr., on Apr. 6, 1965. While uncured thermosettable resins and elastomers can be formed under heat and pressure in a matched metal die mold, they can only be formed before they cure to a thermoset condition. Typically, both heat and pressure must be exerted during the curing reaction to fuse overlapped segments of insulation into a smooth-surfaced, integral layer. For larger solid rocket motors, precured elastomeric material is often used as insulation. This cured material is laid up and joined within a rocket motor casing with an adhesive to fabricate an insulation member. It is then necessary to machine the insulation to provide a smooth surface which does not have overlapped sections. A further disadvantage of using curable resinous or elastomeric insulation is the time required to cure the insulation sufficiently--between several hours and several days.
To alleviate some of the problems of handling thermosetting materials, insulation consisting of filled polyolefins such as polyethylene or polypropylene has been proposed. Besides the obvious fabrication economies of working with thermoplastic insulation, the prior art has recognized the theoretical superiority of thermoplastic resins for ablative insulation because they undergo endothermic pyrolysis, carrying heat away from the insulation. Thermoplastic resins also have high specific heats, and their pyrolysis products have high specific heats and low molecular weights. The theoretical superiority of thermoplastic resins is recognized in U.S. Pat. No. 3,395,035, issued to Strauss on Jul. 30, 1968 (column 6, lines 39-53); and U.S. Pat. No. 3,397,168, issued to Kramer et al., on Aug. 13, 1968 (column 2, lines 15-19; column 3, lines 4-5).
Thermoplastic resin-based material readily melts and flows when subjected to heat. (See the Kramer et al. patent previously cited, column 1, line 64 to column 2, line 4.) Therefore, the art teaches that thermoplastic resins used in ablative insulation must be combined with thermosetting resins and impregnated into a refractory or fiber matrix to prevent the insulation from melting and running off when exposed to the extreme heat and erosion of a rocket motor.
Because insulation represents inert weight of a rocket motor, it would be desirable in some applications to replace high density insulation with lower density insulation of comparable performance.
It would, therefore, be a significant advancement in the art to provide thermoplastic elastomeric ablative insulation materials having low density.
Such low density thermoplastic elastomeric ablative insulation materials are disclosed and claimed herein.